Ischemia/reperfusion (I/R) injury triggers a series of inflammatory responses in the microcirculation characterized by plasma leakage and leukocyte diapedesis. The microvascular pathology largely stems from endothelial barrier dysfunction, a complex cellular process that has not been fully understood at the molecular level. Emerging evidence indicates that the barrier property of the microvascular wall is controlled by dynamic interactions of endothelial cell-cell junctions and cell-matrix focal adhesions; the latter are maily composed of transmembrane integrin receptors and associated proteins such as kindlins and focal adhesion kinase (FAK). These molecules not only provide structural support for endothelial barrier integrity but also transmit biochemical signals that regulate barrier function. The goal of this project is to elucidate the molecular mechanisms underlying FAK-mediated microvascular hyperpermeability during I/R injury. We hypothesize that I/R-elicited oxidative stress upregulates FAK signaling in the microvascular endothelium, inducing integrin (51, v3) internalization and kindlin- targeted -catenin nuclear translocation and junction dissociation, leading to weakened cell-matrix and cell-cell adhesions thereby impairing barrier integrity. This novel pathway will be tested in a series of mechanistic studies centered on intravital microscopic quantification of mesenteric microcirculation, leukocyte dynamics, and fluid/protein permeability in mice subjected to superior mesenteric artery occlusion followed by reperfusion. A newly developed mouse model of endothelial-specific conditional FAK knockout will be used in comparison with pharmacological inhibition of FAK. The in vivo studies will be complemented with imaging analyses and molecular assays in microvascular endothelial cells exposed to hypoxia/reoxygenation or oxidative stress. The studies will provide new mechanistic insights into I/R-induced microvascular injury contributing to the future development of targeted therapies to prevent tissue damage following resuscitation or reperfusion. Knowledge gained from this project may have broad implications in other diseases associated with microvascular barrier injury.